1. Field of the invention
The present invention relates to a method and an apparatus for measuring displacement of molten metal, and particularly to a method and an apparatus for measuring the level of molten metal in a furnace.
2. Description of the Related Art
It is possible to improve the quality of products by controlling the level of molten metal in a mold of a continuous casting furnace to be constant. In order to control the level of molten metal to be constant, there have been proposed and used a variety of systems such as a range finder of the electromagnetic induction system, a range finder of the system using radiation, a range finder of the optical system, and so on. In these systems for measuring distance, in view of their object to control the level of molten metal, particularly the reproducibility has been regarded as important, but the measurement of a large level displacement has not been regarded as very important.
Recently, in the movement of seeking the automatization of equipment and the further improvement of the quality of products, it has come necessary to measure the level of molten metal until the molten metal is established to have a predetermined control level at the beginning of casting. In this measurement of level in the beginning, the displacement of the level is so large that requirements cannot be satisfied in the above-mentioned conventional systems. Accordingly, study and development are being advanced to obtain improvements of those conventional systems or to obtain new systems.
Description will be made under with respect to the conventional systems and the systems which are being developed.
1 Electromagnetic Induction System (Eddy Current System)
By use of a transmission coil (excitation coil) and a reception coil, a magnetic field is generated on the surface of molten metal so that the influence of the magnetic field due to an eddy current generated in the surface of the molten metal is used for measuring the level of the molten metal. This system is high in accuracy of measurement, so that it currently used mainly for controlling the level of molten metal.
2 Thermocouple System
A thermocouple is buried inside a mold, and the level of molten metal is measured on the basis of the temperature change caused by the heat of the molten metal.
3 Ultrasonic Wave System
An ultrasonic wave is radiated into the molten metal, and the distance is obtained on the basis of the time required for the reflected wave to return.
4 Electrode System
An electrode is put into a mold so that the level when molten metal contacts the electrode is outputted.
5 Electromagnetic Wave System
A transmission/reception antenna is provided above a mold so that an electromagnetic wave is radiated toward the surface of molten metal and the reflected wave is received to thereby measure the level of the molten metal on the basis of the propagation time of the electromagnetic wave. This system is disclosed in Japanese Patent Unexamined Publication (JP-A) No. Hei-2-98685, and Japanese Patent Unexamined Publication (JP-A) No. Hei-2-145985.
These systems have problems as follows.
1 Electromagnetic Induction System
Since the displacement of the level of molten metal which can be measured is about 200 mm at the most, it is impossible to measure the level displacement between the level in the beginning of running which displaces more than the above limit of about 200 mm and a predetermined control level. In addition, a protective pipe of ceramics or the like is provided around coils in order to protect the coils from the heat of the molten metal. This protective pipe has a low tolerance against impact. Accordingly, if molten metal adheres to the protective pipe, the protective pipe cannot be used any longer and must be replaced, and the cost is therefore high.
2 Thermocouple System
Responsibility is inferior. In addition, since necessary to bury a thermocouple inside a mold, it costs much to maintain the system.
3 Ultrasonic Wave System
The speed of sound changes depending on temperature, and hence the system receives a large influence from the heat inside the mold. Accordingly, this system has not been put into practical use yet.
4 Electrode System
One electrode can measure only one level position. It is therefore necessary to provided electrodes respectively for various level positions to be measured.
5 Electromagnetic Wave System
Generally, in a small space such as that in a mold, there is an influence of multiple reflection of an electromagnetic wave and it is therefore difficult to measure the level of molten metal. In addition, the space around the mold is small and there is therefore no space to install an antenna therein.